As the flexible engagement gear device, there has been known one that has a flexible external gear of a silk hat shape. In this specification, this type of device is referred to as a silk-hat type flexible engagement gear device.
FIG. 8 is a longitudinal sectional view of a flexible external gear of a silk-hat type flexible engagement gear device, cutting along a plane including its device axial line. As shown in this FIGURE, the flexible external gear 1 has a annular body part 2, a circular diaphragm 3 having an inner decumferential end connected to a proximal-side opening end, and a circular thick boss 5 connected to an outer circumferential end of the diaphragm 3. External teeth 4 are formed integrally on an outer circumferential portion of a distal-side opening end of the body part 2 in a circumferential direction.
The device of this type has an advantage in that a rotational member, various wires and the like can be arranged passing through the device. Namely, since the flexible external gear has the diaphragm 3 extending radially and outwardly from the end of the body part 2, an inner space of the body part 2 can be utilized effectively.
Recently, robots and other machinery have been required to construct compact in size, and therefore miniaturization of reduction device and other mechanisms assembled in robots et al. has also been strongly demanded more than ever. In order to meet this demand, the silk-hat type flexible engagement gear device may be reduced in axial length. For this end, the silk-hat shaped flexible external gear must be reduced in axial length. The conventionally used external gear has a ratio of about 1:1 between the axial length thereof and the pitch circular diameter of the external teeth thereof. If the axial length is made shorter, the coning angle of the external gear is also increased according to the shortening of the axial length.
FIG. 9 illustrates the increase of the coning angle .THETA. of the flexible external gear 1 according to the axial length thereof. As shown in this FIGURE, the increase of the coning angle causes to increase an amount of deformation occurred on portions of the diaphragm 3. As a result, there is a high possibility that an excess stress concentration occurs on the respective portions of the diaphragm 3, especially on the inner end portion 3a and the outer end portion 3b thereof (see FIG. 8).
On the other hand, shearing stress due to transfer torque is also occurred on these inner and outer end portions 3a, 3b of the diaphragm 3, and stress due to misalignment of the component parts is also occurred.
Thus, shortening of the axial length of the flexible external gear tends to cause the excess stress concentration on the portions of the diaphragm 3. For relieving the stress concentration, it is necessary to increase the outer diameter of the diaphragm. It is, however, in the silk-hat type flexible engagement gear device that the size of the outer diameter thereof depends on that of the outer diameter of the flexible external gear. Thus, increase the size of the outer diameter of the flexible external gear causes to increase in size of the device outer diameter. This may be an obstacle to downsizing and compacting of the device.